An illumination unit of this type has been disclosed, for example, in German utility model DE 299 05 836. This publication describes an illumination unit comprising a reflector, an incandescent lamp arranged therein and a reflector neck insert comprising two halves which surrounds the sealed-off end of the lamp vessel and is fixed in the conically formed reflector neck with a clamping fit. The sealed-off end of the lamp vessel has two webs which respectively engage in grooves which are provided on each half of the reflector neck insert.